TheCompositionofSolutions

Molarity •  Tostudysolutionstoichiometry,wemustknowhowmuchofthereactantsarepresentinasolutionandalsohowtocontroltheamountsofreactantsusedtobringaboutareactioninaqueoussolution

•  Concentration=theamountofsolutepresentinagivenamountofsolvent,oragivenamountofsolution

•  Insolutionstoichiometry,weusetheconcentrationasaconversionbetweenamount(volume,g)andmolesofasubstanceinsolution

Molarity •  Molarity(M)=molarconcentration=thenumberofmolesofsoluteperliterofsolution

Molarity •  Youcanalsorewritethistogetthenumberofmoles•  Numberofmoles(n)=Molarity(M)xVolume(L)

Molarity •  Topreparesolutionsofknownconcentration,youwould:

① Massthesolid② Placeinavolumetricflaskofappropriatevolume③ Addenoughdistilledwatertodissolvethesolid④ THENaddwatertothespecifiedmarkontheflask*Ifyoudumpthesolidintotheentirevolumeofwater,

youareneglectingthespacethesolidwilloccupyandyoursolutionwillnotmatchyourconcentration*

Molarity •  StandardSolution=Asolutionwhoseconcentrationisaccuratelyknown– Volumetricflask–containsanexacttotalvolumeofsolution

Molarity •  Ex/Calculatethemolarityofasolutionpreparedbydissolving11.5gofsolidNaOHinenoughwatertomake1.50Lofsolution

Molarity •  Ex/Calculatethemolarityofasolutionpreparedbydissolving1.56gofgaseousHClinenoughwatertomake26.8mLofsolution

Molarity •  Usebalanceddissociationequationtodeterminetheconcentrationofindividualions

•  Inotherwords,wearebasicallymultiplyingionsbytheirsubscript/coefficient

•  Ex/Givetheconcentrationofeachtypeofionin0.50MCo(NO3)2

Molarity •  Ex/Givetheconcentrationofeachtypeofionin1MFe(ClO4)3

Molarity •  Usemolaritytoconvertbetweenmolesandliters•  Ex/CalculatethenumberofmolesofCl-ionsin1.75Lof1.0x10-3MZnCl2

Molarity •  Ex/Typicalbloodserumisabout0.14MNaCl.Whatvolumeofbloodcontains1.0mgNaCl?

Dilution

Dilution•  Concentratedsolutionsareoftenusedas“stock”solutions

•  Thesecanbedilutedtopreparealessconcentratedsolution

Dilution

Dilution•  Equipmentinvolved:

– Pipet/Pipette–toaccuratelymeasureandtransferagivenvolumeofsolution.Maybemeasuring(gradation)orvolumetric(onlyasinglemeasurementvalue)

Dilution•  Ex/Describehowyouwouldprepare5.00x102mLofa1.75MH2SO4solution,startingwithan8.61MH2SO4stocksolution

First:M1= M2=V1= V2=

Dilution•  Ex/Describehowyouwouldprepare5.00x102mLofa1.75MH2SO4solution,startingwithan8.61MH2SO4stocksolution

First:M1=8.61M M2=1.75MV1=? V2=5.00x102mLSecond:PlugintoM1V1=M2V2

Third:Howdoyoupreparethenewconcentration?

Dilution•  Trythis:•  Whatvolumeof16Msulfuricacidmustbeusedtoprepare1.5Lofa0.10MH2SO4solution?

PrecipitationReactions

PrecipitationReactions•  Precipitationreactionsoccurinaqueoussolutionandresultintheformationofaninsolubleproduct(precipitate)

•  Ex/Pb(NO3)2(aq)+2KI(aq)àPbI2(s)+2KNO3(aq)

PrecipitationReactions•  Weoftenseparatetheprecipitatefromthesolutionbyfiltrationusinggravimetricanalysis(moreonthismomentarily)

PrecipitationReactions•  SolubilityRulesàmemorizethecommononesØ MostalkalimetalsaltsandNH4

+saltsaresolubleØ Cl-,Br-,I-aresoluble(exceptforAg+,Hg22+,Pb2+)Ø F-aresoluble(exceptforIIAmetals)Ø NO3

-,ClO3-,ClO4

-,andCH3COO-aresolubleØ SO4

2-aresoluble(exceptforCa,Sr,Ba,Ag+,Pb2+,Hg22+)

Ø CO32-,PO4

3-,C2O42-,CrO4

2-,S2-,OH-,andO2-areinsoluble(exceptforcaseswithrule#1)

Ø Itcanbeassumedthatsaltsthatdissolveinwaterarestrongelectrolytes

PrecipitationReactions•  Wecanpredictwhetherornotprecipitateswillformusingsolubilityrules

•  Ex/Whatprecipitate(ifany)willformifthefollowingpairsofsolutionsaremixed?a.  KNO3andBaCl2b.  Na2SO4andPb(NO3)2c.  KOHandFe(NO3)3

DescribingRxnsinSoln’

RxnsinSoln’s•  Wecandescribereactionsinsolutionsafewdifferentways:

§  Completebalancedequation•  Thisistheoverallreaction

§  Completeionicequation•  Thisrepresentsallsolublecompounds/electrolytesasions

§  Netionicequations•  Spectatorions(ionsthatdidnotchangeorwerenotinvolved)areexcludedandonlythosesolutioncomponentsundergoingachangeareincluded

PrecipitationReactions•  Ex/Whatprecipitate(ifany)willformifthefollowingpairsofsolutionsaremixed?a.  KNO3andBaCl2b.  Na2SO4andPb(NO3)2c.  KOHandFe(NO3)3

GravimetricAnalysis

GravimetricAnalysis•  Gravimetricanalysis=analyticaltechniqueusedtodeterminetheconcentrationofananalyte(anionbeinganalyzed)byprecipitatingtheionasaninsolublesalt,whichcanthenbemassed

GravimetricAnalysis•  Typically,itinvolvestheformation,isolation,andmassdeterminationofaprecipitate

1.  Determinetheanalyte/ionyouneedtoprecipitate2.  Chooseareactionthatwillproduceaninsolublesalt

thatcontainsthation3.  Performaprecipitationrxntocompletelyprecipitate

thetargetionàbesuretoaddexcessoftheothersolutiontoensurecompleteprecipitation(otherwiseyourconcentrationwillbeinaccurate)

4.  Carefully,filter,wash,anddrytheprecipitate5.  Takethemass6.  Usestoichiometrytofigureoutyourconcentration

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GravimetricAnalysis•  Ex/Astudentwantstodetermine[Ba+2]in100.00mLofanunknownsolutionofBa+2.Shedecidedtoprecipitatethebariumionsasbariumsulfate,aninsolublesalt.Sheaddsanexcessofsodiumsulfatesolutiontothebariumsolutionuntilnomoreprecipitateforms.Shethenfilters,washes,anddriestheprecipitate.Shemeasuresthemasstobe2.917g.Determine[Ba+2]inthesolution.

GravimetricAnalysis•  First,assembleyourknowninformation•  Vsoln=100.00mL MassBaSO4=2.917g•  Second,writeyourbalancednetionicequation:

Ba2+(aq)+SO42-(aq)àBaSO4(s)

•  Third,usestoichiometrytodeterminethemolofyouranalyte(Ba2+)

•  Lastly,dividebyvolumeofyoursolutiontogetmolarity

GravimetricAnalysis•  Filters,washes,anddriestheppt,andmeasuresitsmasstobe2.917g

•  Writetheequation:•  Ba+2(aq)+SO4

2-(aq)àBaSO4(s)

•  Makesuretheequationisbalanced•  Convert2.917gBaSO4tomolofBa2+

•  Dividebythevolumeofsolutiontogettheconcentration

GravimetricAnalysis•  Trythis:•  25.00mLofalead(II)nitratesolutionistreatedwithexcessaqueoussodiumsulfate.Afterfiltrationanddrying,0.303gofsolidleadsulfateisisolated.Whatwastheconcentrationofthelead(II)nitratesolution?

GravimetricAnalysis•  Trythis:A0.5662-gsampleofanioniccompoundcontainingchlorideionsandanunknownmetalisdissolvedinwaterandtreatedwithanexcessofAgNO3.If1.0882gofAgClprecipitateforms,whatisthepercentbymassoftheoriginalcompound?

•  Hint,%Cl=massofCl/0.5662gsamplex100%

CW•  Questionpacket

– ShowALLstepsclearly•  StudyCh.4